1. Field of the Invention
The present invention relates to a bearing apparatus for wheel, and more particularly to a bearing apparatus for wheel for rotatably supporting the wheel with respect to a car body.
2. Description of the Background Art
Further improvement in fuel efficiency or running stability of a vehicle such as an automobile has been required in recent years. To that end, a bearing apparatus for wheel rotatably supporting the wheel with respect to a car body has been reduced in size and weight. Consequently, parts constituting the bearing apparatus for wheel tend to be made smaller in size and thickness. Meanwhile, with higher output and performance of the vehicle such as an automobile, it is likely that greater load is imposed on the bearing apparatus for wheel. Under such circumstances, further reduction in size or thickness of parts or improvement in durability of the bearing apparatus for wheel has become difficult.
In order to address such difficulty, it has been proposed to form, in the parts constituting the bearing apparatus for wheel, a hardened portion by subjecting a region insufficient in strength to partial hardening treatment by utilizing induction hardening, to thereby ensure strength, in particular, rotating bending fatigue strength or the like, of that region (see, for example, Japanese Patent Laying-Open No. 2002-087008).
Forming of a hardened portion in a region insufficient in strength, however, may give rise to a problem of strength of a non-hardened portion, which is a portion other than the hardened portion, and it is necessary to additionally form hardened portions. Thus, forming of a large number of hardened portions leads to complicated manufacturing step, in particular, heat treatment step, of the bearing apparatus for wheel, and manufacturing cost is increased. In addition, if a thin portion having a small thickness or a region adjacent to the thin portion is subjected to hardening treatment such as induction hardening, another problem of deformation of the thin portion due to heat treatment may arise. Meanwhile, it may be effective to harden the part, of which strength is required, in its entirety, among the parts constituting the bearing apparatus for wheel, to thereby eliminate a non-hardened portion and ensure strength. If the part is formed as the hardened portion in its entirety, however, subsequent working becomes difficult, and it is not necessarily effective to adopt such measures.